The present invention relates to a treatment of a reaction mixture of sucrose and a fatty acid alkyl ester. More particularly, the present invention relates to an industrially useful process for recovering the unreacted sucrose from the reaction mixture obtained in the synthesis of sucrose fatty acid esters in an organic solvent medium.
Sucrose fatty acid esters (sugar esters) useful as surface active agents are prepared industrially at present by either a solvent process wherein sucrose is reacted with a methyl ester of a higher fatty acid having 8 to 22 carbon atoms in the presence of a suitable catalyst in an organic solvent such as dimethylformamide or dimethylsulfoxide, as disclosed in Japanese Pat. Publication Kokoku No. 35-13102; or an aqueous medium process wherein sucrose is formed into a molten mixture with a fatty acid salt (soap) using water without using an organic solvent, and is then reacted with a higher fatty acid methyl ester in the presence of a catalyst, as disclosed in Japanese Pat. Publication Kokoku No. 51-14485. However, even according to any of these processes, the obtained reaction mixture contains impurities such as the unreacted sucrose, the unreacted fatty acid methyl ester, residual catalyst, soap, free fatty acid, volatile material, etc. in addition to the desired sucrose fatty acid ester. These impurities, at least impurities whose contents exceed the specified amounts must be removed prior to being put on the market. Purification according to conventional processes requires a large amount of an organic solvent as purification solvent. On the other hand, removal of the solvent (volatile material) remaining in the product is very important in view of strict regulation, e.g. provision by FDA, U.S.A according to which allowable content of remaining dimethylsulfoxide in sucrose fatty acid esters is at most 2 p.p.m. [Fed. Regist., 51(214), 40160-1]. Removal of the unreacted sucrose is also important because it is included in a large amount in the reaction mixture.
The use of a large amount of solvents conventionally used for purification of sucrose fatty acid esters, in particular, upon industrial production of sucrose fatty acid esters, results in the following disadvantages: (1) risk of explosion and fire, (2) provision of explosion and fire prevention means to electric devices, (3) application of closed system to production equipment for explosion and fire prevention, (4) requirement of fireproof construction for entire building by way of precaution against explosion and fire, (5) rise in fixed cost due to the items (2), (3) and (4), (6) rise in materials cost due to loss of solvent, (7) contamination of the product with remaining solvent, and (8) adverse influence on health of workers, and increase of cost resulting from increase in labor required for the prevention therefor.
In view of these circumstances, it has been desired to develop a purification technique capable of removing the unreacted sucrose and other impurities from the crude reaction mixture without using organic solvents.
Thus, a purification method using no organic solvent has hitherto been studied. For example, as a representative method, there has been known (1) a method wherein a sucrose fatty acid ester is precipitated by addition of an acidic aqueous solution to the reaction mixture, as disclosed in British Pat. No. 809,815 and (2) a method wherein a sucrose fatty acid ester is precipitated by addition of an aqueous solution of a common neutral salt to the reaction mixture, as disclosed in Japanese Pat. Publication Tokkyo Kokoku No. 42-8850.
However, these methods have disadvantages. When an acidic aqueous solution, for example, hydrochloric acid, is added to the reaction mixture as in the method (1), the sucrose fatty acid ester immediately deposits, but the unreacted sucrose is easily decomposed and converted into glucose and fruit sugar. This cannot be avoided even if the addition is conducted at a low temperature (e.g. 0.degree. to 5.degree. C.). Accordingly, the recovery and reuse of the unreacted sucrose become difficult. As is well known, the conversion of sucrose in the synthesis of sucrose fatty acid esters is low. For example, even in the synthesis using dimethylformamide, the conversion of sucrose is at most 50% and, therefore, the recovery of unreacted sucrose is essential from the economical point of view.
Also, the addition of an aqueous solution of a neutral salt such as sodium chloride or Glauber's salt, as in the method (2), causes sucrose fatty acid esters to deposit rapidly. In this case, decomposition of unreacted sucrose does not occur, but the monoester which is an effective component in the product is dissolved in an aqueous phase. Consequently, not only the dissolution results in a large loss of the product, but also it is a hindrance particularly to production of sucrose fatty acid esters having a high HLB which are recently in great demand.
Also, in Japanese Pat. Publication Tokkyo Kokai No. 51-29417, it is proposed to utilize a property that a mixture of water and a solvent used for purification (hereinafter referred to as "purification solvent") separates into an upper light layer and a lower heavy layer. Generally, the lower layer contains a large amount of water and, therefore, the unreacted sucrose, which is hydrophilic, and a salt derived from a catalyst used in the synthesis of sucrose fatty acid esters are dissolved in the lower layer. Since the upper layer contains the purification solvent in a large quantity, compounds having a small polarity such as sucrose fatty acid esters, fatty acids and unreacted fatty acid methyl esters are dissolved in the upper layer. On the other hand, the solvent used for the reaction such as dimethylsulfoxide is dissolved not only in the lower layer, but also inconveniently in the upper layer. Consequently, it is impossible to completely separate the reaction solvent only by this method. In addition, not only the product is contaminated with a trace amount of the reaction solvent, but also a very large amount of the purification solvent is required for removing the unreacted sucrose from the reaction mixture.
In order to industrially realize the purification of crude sucrose fatty acid esters with water, it is very important that the removal of the reaction solvent and the purification solvent is complete and moreover sucrose and the product are not lost.
Another important problem which must also be taken into consideration is a means for recovering the unreacted sucrose incident to the use of water as a purification solvent. Since the purification of the reaction mixture with the use of water is based on difference in water solubility between a sucrose fatty acid ester and unreacted sucrose, migration of a large amount of unreacted sucrose into an aqueous phase cannot be avoided. The manufacture of sucrose fatty acid esters cannot be industrially accepted unless such a dissolved sucrose is recovered. Accordingly, it is also very important to efficiently recover the sucrose which has migrated into an aqueous phase upon purification.
It is a primary object of the present invention to provide a method for recovering unreacted sucrose from the reaction mixture, while removing the reaction solvent without using organic solvents for the purification of the reaction mixture.
The above and other objects of the present invention will become apparent from the description hereinafter.